This invention relates to electronic apparatus for surface-mounting and surface mounting devices for such apparatus. More particularly, this invention relates, on one hand, to electronic apparatus adapted for surface mounting such as thermistors for controlling a current and, on the other hand, to surface mounting devices for such an electronic apparatus having a pair of electrodes on its upper and lower surfaces.
Consider, for example, a positive temperature coefficient (PTC) thermistor element as shown at 101 in FIG. 12, as an example of a planar ceramic electronic element having a pair of ohmic electrodes 101a and 101b on its lower and upper surfaces. The PTC element may be a disk with thickness 3 mm and diameter 6 -10 mm. A prior art method of mounting such an element to a printed circuit board was to provide connector electrodes (say, of silver) on the ohmic electrodes 101a and 101b, to attach lead lines to these connector electrodes and to attach the element to the circuit board through these lead lines. For providing compact apparatus, however, such elements are now mounted directly to the circuit board.
For this purpose, a printed circuit board 104 as shown in FIG. 12 with contact electrodes 104a and 104b may be provided, cream solder 103 may be applied over the circuit board electrode 104a before the element 101 is placed thereon, and its ohmic electrode 101a on the lower surface is connected to the contact electrode 104a by reflow soldering. Next, one end of an electro-conductive lead line 105 is soldered to the other electrode 101b on the upper surface of the element 101, the other end of the lead line 105 being soldered to the other contact electrode 104b. In FIG. 12, numeral 106 indicates the solder used for the attachment of the lead line 105.
Surface-mounted prior art elements of this kind have many problems. Firstly, two kinds of soldering are required for the mounting, the reflow soldering by which the element 101 is connected to the circuit board 104 and the soldering of the lead line 105 to the circuit board 104 and to the element 101. This makes the mounting a time-consuming process. Secondly, since it is not practically possible to simultaneously carry out the soldering of both electrodes 101a and 101b on the two principal surfaces of the element 101, the heat from the first soldering of the lower-surface electrode 101a affects adversely the soldering of the upper-surface electrode 101b. The situation would not change if the order of soldering were reversed, because the soldering characteristics of the electrode to be soldered later are adversely affected. Thirdly, since the ceramic element 101, which is sensitive to heat, is heated twice, micro-cracks are likely to be generated due to the thermal stress. Fourthly, since the element 101 generates heat when it is operated, the generated heat is directly transmitted to the circuit board 104 and through the circuit board 104 to other electronic elements which may be mounted thereon, adversely affecting their performance.